The present invention relates to a magnet roller adopted for a developing roller which is used, in an electrophotographic apparatus or an electrostatic recording apparatus such as a copying machine or a printer, for supplying a developer to an image forming body holding an electrostatic latent image, for example, a photosensitive drum, a belt, or a sheet of paper, OHP or photographic paper and forming a visible image on the surface of the image forming body, and also relates to a developing roller composed of the magnet roller and a sleeve. In particular, the present invention relates to a magnet roller composed of a magnet main body and shaft portions formed integrally with the magnet main body wherein the magnet roller is high in roundness of each shaft portion and is thereby rendered highly noiseless upon operation, and also relates to a highly noiseless developing roller using the above magnet roller.
Conventionally, for an electrophotographic apparatus or an electrostatic recording apparatus such as a copying machine or a printer, there has been adopted a developing method using, as a developing roller for visualizing an electrostatic latent image on a latent image holding body such as a photosensitive drum, a magnet roller which is formed from a bond magnet and is disposed in a rotating sleeve. In this method, a magnetic developer (toner) supported on the surface of the sleeve is supplied on the surface of the latent image holding body by a so-called jumping phenomenon allowing the toner to be jumped on the latent image holding body by a magnetic force characteristic of the magnet roller, to thereby visualize the electrostatic latent image.
The above magnet roller has been conventionally manufactured by a manner of mixing a magnet powder made from ferrite or the like with a binder composed of a thermoplastic resin such as nylon or polypropylene to prepare pellets of a bond magnet composition, and injecting-molding or extrusion-molding these pellets of the bond magnet composition using a die applied with a magnetic field, to thereby form the bond magnet composition into a roller shape and also magnetize the bond magnet composition with a specific magnetic characteristic. In this case, shaft portions for supporting the magnet roller have been generally provided at both ends of the magnet roller, and to reduce the manufacturing cost and enhance the magnetic force, there has been proposed a method in which each or either of both the shaft portions and the magnet main body have been integrally molded from a bond magnet composition.
The magnet roller of the type in which the magnet main body and each or either of the shaft portions are integrally molded from a bond magnet composition, however, has a problem that noise is liable to occur upon developing operation. In particular, for a developing roller of a type in which not only the sleeve but also the magnet roller is rotated, there is a tendency that noise significantly occurs.
To be more specific, there is known a developing roller of a type shown in FIG. 1, in which two shaft portions 12 of a magnet roller 1 are rotatably supported by two bearings 3 mounted in a sleeve 2 rotatably disposed so that the magnet roller 1 is rotatably disposed in the sleeve 2, wherein developing operation is performed by rotating the magnet roller 1 in the direction reversed to the rotational direction of the sleeve 2. In this case, if the magnet roller 1 of the developing roller is composed of a magnet main body 11 formed integrally with the shaft portions 12, there occurs noise upon operation, and particularly, for a developing roller of a type in which the above shaft integral type magnet roller 1 is rotated at a high speed of 150 rpm or more, there significantly occurs noise.